sequencer: run 'prepare-commit-msg' hook

Commit 356ee4659b ("sequencer: try to commit without forking 'git
commit'", 2017-11-24) forgot to run the 'prepare-commit-msg' hook when
creating the commit. Fix this by writing the commit message to a
different file and running the hook. Using a different file means that
if the commit is cancelled the original message file is
unchanged. Also move the checks for an empty commit so the order
matches 'git commit'.

Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.h

@@ -1,6 +1,7 @@
 #ifndef SEQUENCER_H
 #define SEQUENCER_H
 
+const char *git_path_commit_editmsg(void);
 const char *git_path_seq_dir(void);
 
 #define APPEND_SIGNOFF_DEDUP (1u << 0)